In the cultivation of fruit trees of various kinds, it is essential to make preliminary works for picking off some bodies of flower or fruit which are not required to fructify. Such works for picking off the unrequired bodies of flower or fruit are known as "thinning" of flower or fruit. Meanwhile, such thinning of flower or fruit usually needs a large amount of labor by the workers. The thinning works usually must be done in such seasons of busy work in agriculture, and hence it is frequently difficult to make the thinning works at an appropriate time therefor, so that the quality of the fruits cultivated and harvested could be lowered sometimes.
Heretofore, the methods for thinning the flower or fruit of fruit trees by chemical compounds have been extensively researched for almost all kinds of fruit trees such as apple tree, peach tree and others, and there have been proposed and developed a number of chemical thinning agents. The known chemical thinning agents may be classified according to their biological mechanisms of the action of these agents, and they may largely be classified into two groups, that is, a first group of such agents which act as a plant hormone and the second group of such agents which can exert a phytotoxicity against the bodies or different organs of flower of the fruit trees. While, such chemical thinning agents which have been extensively applied in practice for partial removal of the unrequired flowers or fruits are limited only to the formulation known as the name "Ethyclozate" for use in the thinning treatment of tangerine tree; the formulation known as the name "Sevin" for use in the thinning treatment of apple tree; and certain sulfur preparations for use in the thinning treatment of apple tree. The active components of the thinning agents known under the names "Ethyclozate" and "Sevin" are belonging to a class of plant hormone in nature, so that their flower-thinning effects can unfavorably vary depending on the biological conditions of the trees treated and also on the weathering conditions, which is specifically observed with the plant hormone agents in usual. On the other hand, the sulfur preparations practically employed as the chemical thinning agent are active to inhibit the fertilization of flowers owing to its phytotoxicity to the stigma of the flower body, so that its flower-thinning activity is normally lower than those of the flower-thinning agents of plant hormone type. When the sulfur preparations are applied in an increased rate of application in an attempt to enhance its flower-thinning effects, the leaves and other portions of the fruit tree as treated can often be damaged by the phytotoxicity of the sulfur preparations.
For these reasons, there exists a great demand for a new flower-thinning chemical agent which is free from the above-mentioned drawbacks of the known chemical flower-thinning agents. In an attempt to meet this demand, we have researched a number of chemical compounds which are different in nature from the active components of the known thinning agents, and we have now found that 2-pyrrolidone-5-carboxylic acid and salt thereof such as the sodium salt; and salt of an N-higher aliphatic acylated acidic amino acid; as well as an ester of sugar with a higher fatty acid and plant sterols are each active to thin the flowers of the cultivated fruit trees.